The simultaneous combustion of two or more fuel types to provide useful energy. Generally, a primary fuel is combusted with one or more supplemental fuels. Examples would include the co-combustion of wood with coal, or processed combustible materials derived from residential, commercial and industrial sources, which could include plastics-enhanced pelletized fuel products, with coal as the primary fuel in industrial or utility boilers. (Kenneth Smith, Vice President, wTe Corporation, Bedford, Mass., 1996).